Like humans, dolphins have two hemispheres in their brain. But while we rest the whole of our brain (and body) through sleep at the same time, for dolphins it is not so easy as all that.
How do dolphins sleep?
Dolphins have to come to the surface when they rest in order to breathe, which they do consciously (ie by choice). If they didn’t surface, they could drown.
They also have to be alert to changes in local conditions and to the presence of predators such as sharks, or even humans.
So what evolutionary trick must they perform to get the sleep they need?
Dolphins close the left eye when the right half of the brain sleeps, and vice versa, alternating which half of the brain is sleeping so that they can rest without losing consciousness.
Dolphins often lie motionless at the surface, breathing regularly, or swim slowly close to the surface.
In shallow water, they might sleep on the seabed, rising regularly to breathe. So, they do sleep, but not like we do.
Main image: Atlantic spotted dolphin, Stenella frontalis © Getty Images
